1. Field of the Invention
The present invention relates to a process for the direct reduction of iron-containing material, wherein synthesis gas, preferably reformed natural gas, is mixed with top gas forming in the direct reduction of the iron-containing material and is used for direct reduction as a reducing gas which does not crack at reduction temperature, as well as a plant for carrying out the process.
2. Description of the Prior Art
A process of this type is known, for instance, from U.S. Pat. No. 2,752,234, U.S. Pat. No. 5,081,251, and EP 0 571 358.
From EP 0 571 358 it is known to carry out the reduction of fine ore not exclusively via the strongly endothermic reaction with H.sub.2 according t o EQU Fe.sub.2 O.sub.3 +3H.sub.2 =2 Fe+3H.sub.2 O-.DELTA.H,
but additionally via the reaction with CO according to EQU Fe.sub.2 O.sub.3 +3CO=2 Fe+3CO.sub.2 -.DELTA.H,
which is an exothermic reaction. Thereby, it is feasible to considerably lower the operational costs involved.
However, in metallic plant parts that get into contact with CO-containing reducing gas are subject to a high stress due to corrosion. The result is metal decomposition, which is denoted as "metal dusting" in the technical literature. Metal dusting occurs to an increased extent at elevated temperatures, plant parts that get into contact with hot CO-containing reducing gas, thus, being particularly jeopardized. With a plant for carrying out the initially mentioned process, these are primarily the reactors employed for direct reduction and the gas heater heating the reducing gas to reduction temperature.
With a process according to the prior art, difficulties may occur when the reducing gas has a water content that is either too high or too low. If the water content is too high, the reduction potential of the reducing gas decreases. As a result, a higher amount of reducing gas is required for direct reduction than would actually be necessary in accordance with the chemical potential of the reducing gas. A further difficulty arises if the water content of the reducing gas is too low, because too dry a reducing gas causes metal dusting to an increased extent. In order to minimize both of these drawbacks, it is necessary to adjust the optimum water content with great accuracy.